Aerospace honeycomb core sandwich panels (having composite laminate skins co-cured with adhesives to the core through autoclave processing) find widespread use today because of the high stiffness-to-weight (i.e., "specific stiffness) and strength-to-weight (i.e., specific strength) ratios the panels afford. Typical honeycomb core sandwich panels are described in U.S. Pat. Nos. 5,284,702; 4,622,091; and 4,353,947, which we incorporate by reference. Alteneder et al., Processing and Characterization.sub.-- Studies of Honeycomb Composite Structures, 38th Int'l SAMPE Symposium, May 10-13, 1993 (PCL Internal No. 200-01/93-AWA) discusses common problems with these panels, including core collapse (i.e., core crush), skin laminate porosity, and poor tool surface finish. We incorporate this article by reference.
With a high flow resin system, large amounts of resin can flow into the core during the autoclave processing cycle. Such flow robs resin from the laminate, introduces a weight penalty in the panel to achieve the desired performance, and forces over-design of the laminate plies to account for the flow losses. The resin loss from the laminate plies also reduces the thickness of the cured plies which compromises the mechanical performance. To achieve the desired performance and the corresponding laminate thickness, additional plies are necessary with resulting cost and weight penalties. Because the weight penalty is severe in terms of the impact on vehicle performance and cost in modem aircraft and because the flow is a relatively unpredictable and uncontrolled process, aerospace design and manufacture dictates that flow into the core be eliminated or significantly reduced. In addition to the weight penalty from resin flow to the core, we discovered that microcracking that originated in the migrated resin could propagate to the bond line and degrade mechanical performance. Such microcracking potential poses a catastrophic threat to the integrity of the panel and dictates that flow be eliminated or, at least, controlled.
Flow from the laminates to the core occurs because of viscosity reduction of the resin (i.e., thinning) at the elevated processing temperatures. Therefore, prior art attempts to solve the flow problem have generally focused on retaining the ambient temperature viscosity of the resin at the curing temperatures. For example, we might alter the processing cycle to initiate curing of the resin during a slow heat-up, low pressure step to induce resin chain growth before high temperature, high pressure completion. In this staged cure cycle, we would try to retain the resin's viscosity by building molecular weight at low temperatures. Higher molecular weight resins have higher inherent viscosity so they remain thicker and are resistant to damaging flow to the core. Unfortunately, with a staged cure cycle, too much flow still occurs, and the potential problems of microcracking still abound. Also, facesheet porosity might increase beyond acceptable limits. Furthermore, a modified cure cycle increases autoclave processing time. Increased processing time translates to a significant fabrication cost increase with risk of rejection of high value parts at the mercy of uncontrolled and inadequately understood factors.
U.S. Pat. No. 5,445,861 describes sound absorbing composite sandwich structure. The sandwich structures have seven layers as follows:
(1) an outer skin;
(2) a small celled honeycomb or foam core;
(3) a frontside inner septum;
(4) a large celled middle honeycomb core;
(5) a backside, inner septum;
(6) a backside, small celled honeycomb or foam core; and
(7) an inner skin.
We often place tuned cavity absorbers in the middle honeycomb core. Performance of this structure suffers from resin flow to the cells of the honeycomb cores during fabrication for the reasons already discussed and because such flow alters the resonance of the structure. We incorporate this patent by reference.
The process of the present invention eliminates resin (matrix) flow into the honeycomb core for sandwich structure using high flow resin systems and results in reproducibility and predictability in sandwich panel fabrication and confidence in the structural performance of the resulting panel.